1. Field of the Invention
The invention relates to optical communications systems comprising both wavelength-monitoring and polarization-monitoring devices.
2. Discussion of the Related Art
Optical communications systems utilizing wavelength division multiplexing (WDM) have become more prevalent and important, as the need and desire to send and receive data has rapidly increased. Stated briefly, WDM systems operate by simultaneously sending data on numerous channels, i.e., wavelengths. During operation of the systems, it is generally necessary to monitor the status of these wavelengths, e.g., ensure that a channel is present and/or is of sufficient intensity.
One technique for such wavelength monitoring is the use of fiber grating-based spectrometers, as discussed for example in co-assigned U.S. Pat. Nos. 5,850,302 and 6,002,822 and co-assigned U.S. patent application Ser. No. 09/093,323 filed Jun. 8, 1998 (our reference Koeppen 1-11), the disclosures of which are hereby incorporated by reference. These spectrometers, in general, utilize an optical fiber having a blazed, i.e., tilted, Bragg grating therein, to direct at least a portion of the propagating light out of the fiber and onto a detector. Selection of particular grating characteristics and/or use of optics induces light of differing channels to impact the detector at distinct locations, allowing the presence/intensity of the different channels to be monitored. Because these spectrometers are fiber-based, it is possible to incorporate them in-line. (In-line indicates that the device is able to be incorporated into the fiber transmission line of a communications system without unduly hindering the system.)
In addition to wavelength monitoring, it is generally desirable to monitor the state of polarization (SOP) of a propagating optical signal, due to the inherent degeneracy associated with polarization. Polarization measurement is important, for example, in accurately orienting polarization maintaining fiber during splicing, in measuring the polarization dependent loss (PDL) of components or system, and in determining the polarization-mode dispersion (PMD) in optical transmission systems. The significance of PMD is increasing due to the development of ever faster-speed, long-haul systems.
Most conventional arrangements for polarization monitoring rely on statistical sampling of polarization states. See, e.g., U.S. Pat. No. 5,440,390. However, real-time monitoring is desirable, particularly if the monitoring can be done in-line. Such in-line systems are reflected in co-assigned U.S. patent application Ser. No. 09/517,865 filed Mar. 3, 2000 (our reference Erdogan 11-38-4) and co-assigned U.S. provisional patent application Ser. No. 60/187,840 filed Mar. 8, 2000 (our reference Erdogan 12-42-7), the disclosures of which are hereby incorporated by reference. The in-line fiber grating polarimeter disclosed in these applications involves use of a series of strongly-blazed Bragg gratings that both tap light from the propagating signal and also constitute polarization sensitive elements. Significantly, provisional patent application 60/187,840 contemplates combining the in-line polarimeter with elements to perform wavelength separation, which would allow one to monitor both wavelength and polarization. See also U.S. patent application Ser. No. 60/228,265 filed Aug. 25, 2000 (our reference Moeller 9-12) for other ways to measure polarizaton states.
Further improvements in both wavelength and polarization monitoring are desired.
The invention provides a combined spectrometer/polarimeter device capable of being placed in-line in an optical transmission system, typically a WDM system. The combined device contains an optical waveguide, wavelength manipulating optics that include one or more wavelength dispersive elements formed in the waveguide, and polarization manipulating optics. The wavelength dispersive elements tap at least a portion of the propagating light from the waveguide, such that the tapped light is directed to or through at least a portion of the polarization manipulating optics. As in the co-assigned patents and application cited above, the presence of the wavelength dispersive element(s) allow monitoring of one or more wavelengths present in the propagating light. Thus, the device is able to act as a spectrometer. But, in addition, by inclusion of the polarization-manipulating optics, it is further possible to use the same in-line device to monitor the polarization for each such wavelength and/or for particular channels (e.g., calculation of a Stokes parameter spectrum or measurement of PMD).
In one embodiment, reflected in FIGS. 1A and 1B, an optical fiber 10 having a core 12 and a cladding 14 is provided, with a blazed grating 16 formed in the fiber by conventional techniques. The grating reflects, i.e., taps, light (over a range of wavelengths) from the fiber through coupling opticsxe2x80x94which comprise in this embodiment an index matching medium 18 a glass block 20, and a reflective element 22xe2x80x94toward a detector array 24. As discussed in the references cited above, in such a configuration, tapped light of wavelength xcexi is brought to a focus at some point or along some line on the detector 24 distinct from the point or line of xcexjxe2x89xa0xcexi. With use of a proper array, the multiple wavelengths are able to be resolved and detected simultaneously over a relatively large bandwidth, e.g., 20 to 50 nm. The configuration is thereby able to act as an in-line spectrometer of the type discussed above. However, according to the invention, the device also provides polarization-manipulating optics 25. In the embodiment of FIGS. 1A and 1B, the necessary polarization optics, e.g., optics 26, 27, 28, 29, are provided between the coupling optics and the detector 24, to allow, for example, calculation of the Stokes parameter. Advantageously, the light tap of this and similar embodiments has substantially no effect on the polarization of the scattered (tapped) light, i.e., the tap induces less than 1 dB of variation in the scattered light as a function of polarization. Also, to keep PDL low, it is advantageous to keep the transmission line free of linear polarizers, and provide dispersive elements that introduce relative low PDL, e.g., less than 0.2 dB.
In another embodiment, reflected in FIG. 6, the system contains a fiber 100 having a portion of polarization manipulating optics (e.g., a four-state polarization controller 102) located in-line, upstream of a blazed grating 104 formed in the fiber 100. Scattered light travels through a lens 106 onto a detector 110. The blazed grating in this embodiment acts both as a wavelength insensitive tap for transmitted light and as a polarizer for scattered light (i.e., as another portion of the polarization monitoring optics). Thus, no distinct polarizer is required between the tap 104 and the detector 110. (Note that the functioning of the tap as a polarizer for scattered light brings the system within the above definition of having the tapped light directed to and/or through at least a portion of the polarization manipulating optics.)
(Wavelength manipulating and polarization manipulating optics include optics performing functions such as manipulating or controlling the polarization or selected wavelengths to allow the ultimate monitoring of wavelength or polarization. Wavelength manipulating optics include, for example, in addition to the wavelength dispersive elements, lenses and reflective elements capable of directing different wavelengths to different regions on a detector. Polarization manipulating optics include, for example, linear polarizers, retardation plates such as quarter- and half-waveplates, four-state polarization controllers, rotating retardation plates, and polarization switches. It is possible for single element to be both wavelength manipulating and polarization manipulating, e.g., providing dual functions as in a highly blazed grating that provides a substantially wavelength-insensitive tap for transmitted light and which also provides a polarizer for scattered lightxe2x80x94see FIG. 6 and accompanying discussion. Detectors, typically used to allow monitoring of the presence/strength of particular wavelengths, and to allow monitoring of the polarization states of such wavelengths, are not considered herein to be manipulating optics, unless the detector comprises an integral component that manipulates a wavelength or polarization state, e.g., a birefringent coating or layer.)